In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated by a pre-patient collimator to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts that attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To generate a high quality image, the x-ray source position during a CT scan is required to be known with a very high precision. In at least one known CT system, gravitational and thermal effects cause focal spot motion, i.e., motion of the x-ray source in the z-axis relative to the detector and pre-patient collimator. One component of the focal spot motion causes the fan beam to move along the z-axis. Specifically, focal spot motion displaces the fan beam at the detector location according to the ratio of collimator to detector distance and collimator to focal spot distance. This ratio is typically larger than one. The fan beam movement, or displacement, if uncorrected, results in image artifacts and otherwise degrades image quality.
In known CT systems, dedicated detectors with z-resolution, e.g., z-wedge detectors, are used to detect displacement of the fan beam along the z-axis. Such detectors are outside the fan beam area that is introduced to the object to be scanned. As a result, such detectors are always exposed to the unattenuated beam. Z-wedge detectors require that the detector z-dimension be larger than the fan beam z-dimension. Any fan beam movement in the z-direction then affects the intensity of the signal output by the detector. By monitoring the signal intensity at the z-wedge detector, fan beam motion, and thus focal spot motion, can be detected.
However, known z-wedge detectors and other known position sensitive structures are ineffectual if the entire surface area of the detectors is flooded by the fan beam. Particularly, if the fan beam floods the entire z-wedge detector cell, even during z-axis displacement, the z-wedge detector cell output signal remains constant. Under such conditions, a z-wedge detector is unable to determine focal spot motion.
It is desirable to detect fan beam movement with high accuracy so that a high quality image with a low level of artifacts can be generated. It also is desirable to detect such fan beam movement even if the fan beam floods the entire detector cell surface.